Process for forming a seal for a container for storing a liquefied gas



July 19, 1966 E. M. SCHLUMBERGER PROCESS FOR FORMING A SEAL FOR A CONTAINER FOR STORING A LIQUEFIED GAS Filed Feb. 28. 1965 FIG. I.

'1 P 2 11 v r 3/ 1 r \4gv 1 'E x FIG. 2.

Efienne M.

ATTORNEY FIG. 5.

Fix Blocks 4 ToBacking 3 LeavinF GopBefween B ocks Compress Slabs 7 To Width Of GapAndApply Adhesive For Sealing Slabs To Blocks Insert Compressed Slab Into GapAndAllow ltTa Expand AfAmbiemTerrp At Cryogen cTemperafu Contraction Will Put Slabs Under Tension/As Gap Increases INVENTOR Schlumberger United States Patent Office 3,261,687 Patented July 19, 1966 3,261,087 PROCESS FOR FORMING A SEAL FOR A CON- TAINER FOR STORING A LIQUEFIED GAS Etienne Maurice Schlumberger, London, England, as-

signor to Conch International Methane Limited, Nassau, Bahamas, a Bahamian company Filed Feb. 28, 1963, Ser. No. 261,888 Claims priority, application Great Britain, Mar. 6, 1962, 8,653/ 62 18 Claims. (Cl. 29-450) The invention relates to a process for forming a fluidtight seal for use in a container for storing liquefied gases at low temperatures, especially for use in a container for storing liquefied natural gas, methane, nitrogen, oxygen, ethane, propane or air at about atmospheric pressure.

Containers for storing the above-mentioned liquefied gases usually comprise a rigid 'outer shell internally lined with heat-insulating material and an inner tank in the space enclosed by the heat-insulated outer shell. The temperatures of the liquefied gases are very low indeed; the temperature of liquefied natural gas is, for example, as low as minus 161 degrees centigrade, consequently, the temperature of the inner tank will vary between ambient temperature when the inner tank is empty and about the temperature of the liquefied gas when the inner tank is loaded. This temperature variation will cause large expansions and contractions of the material of the inner tank and of the heat-insulating material adjacent to the inner tank. The heat-insulating material lining the inner surfaces of the rigid outer shell consists normally of blocks of heat-insulating material (e.g. closed cell foamed materials) which are secured independently of each other to the rigid outer shell. The rigid outer shell is normally made of a material which will lose its strength and ductility if cooled down to the low temperature of the liquefied gas, therefore measures must be taken to prevent the cold liquefied gas from ever reaching the rigid outer shell. It will be understood that it is desirable to construct the layer of heat-insulating material in such a way that liquefied gas or vapor cannot penetrate it so that, in the case of failure of the inner tank, the liqeufied gas will not be able to contact and to cool down the rigid outer shell. This means that when blocks of heat-insulating material are used, it is necessary to seal the joints between adjacent blocks in such a manner that, although the blocks expand and contact substantially, the liquid-tightness of the seal is maintained. Therefore, there are often voids between the panels or blocks to provide the necessary space for thermal contractions and expansions.

If foamed plastics are installed (e.g. in these voids) at ambient temperature and then cooled down to about the temperature of the liquefied gas, only a limited extension of the foamed plastic is possible at the low temperature, since the elastic properties of the foamed plastics decrease when they are cooled down substantially. Now, the applicant has found that if foamed plastic, in particular polyvinyl chloride, is installed at ambient temperature in precompressed condition a significant extension of the foamed plastic is possible at low temperature before breakage will occur.

Use can be made of the above-mentioned property of foamed plastics in constructing in a simple way a fluidtight seal between adjacent blocks of heat-insulating material in a container for storing liquefied gases at low temperature.

According to the invention, a process for forming a seal which is fluid-tight at low temperatures is one in which the zone to be sealed is substantially filled with a compressible closed cell porous material at ambient temperature and in the precompressed condition, the closed cell porous material being bonded firmly to the material bounding the zone.

The specific nature of the invention as well as other objects and advantages thereof will clearly appear from a description of a preferred embodiment as shown in the accompanying drawing, in which:

FIG. 1 is a sectional view of a portion of a tank according to the invention, showing the novel construction;

FIG. 2 is a sectional view showing the liner when the container is at ambient temperature;

FIG. 3 shows the liner when the container is at low temperature;

FIG. 4 indicates the initial thickness of the liner slab at ambient temperature;

FIG. 5 is a flow chart of the method.

As shown in FIG. 1, the container 1 comprises a rigid outer shell 3 made of a material which will lose its strength and ductility if cooled down to the low temperature of liquefied gas; this may typically be the inner steel hull of a ship. Suitably fixed to the outer shell 3 as by pins 6 are blocks 4 of rigid insulating material such as balsa wood, cork-board, and so forth, which may if desired be enclosed in plywood. After the blocks are put in place, and before the inner lining 2 of the container is added, sealing slabs 7 are inserted in pre-compressed condition between adjacent blocks 4. FIG. 4 shows the unstressed thickness of slab 7 as indicated by the letter w. As shown in FIG. 2, this is compressed by between 5% and 25% of it original thickness when inserted between adjacent blocks 4 at ambient temperature. adhesive, as explained below, to the adjacent blocks 4. As shown in FIG. 3, when subjected to the low temperature of liquefied gas, both the blocks 4 and the slab 7 tend to shrink away from each other, but due to the ad hesive bonding between them, the result is that the slab 7 is placed in a state of tension without losing its seal.

The actual amount of expansion and contraction is of.

course very small and is greatly exaggerated in the drawings for the purpose of illustration.

By closed cell porous material is meant a material which is predominately closed-cell, it being difficult in practice to obtain a material which is completely closedcell. Some types of P.V.C. foam are always predominately closed cell structure. Of course the compressible closed cell porous material itself should be fluid-tight. The compressible closed cell porous material should preferably be a synthetic material such as plastic material. When using a foamed plastic it should preferably be a thermoplastic. Examples of suitable closed cell foamed plastics are polystyrene, polyethylene and polypropylene or polyurethane foams or especially polyvinyl chloride. Preferably urea foams are not used. As an alternative to thermoplastic foams, closed-cell elastomers may be used.

The compressible closed cell porous material should preferably be semi-rigid, that is a material able to be compressed at least 5% without serious damage to the structure. Preferably the semi-rigid material is one having at ambient temperature a compressive strength at 20% compression of between 5 and p.s.i. (0.35 and 5.6 kg./cm. If the closed cell porous material is too soft there is very little elongation at low temperatures,

The slab 7 is adhesively bonded by a suitable- 25 whereas if it is too hard, the cell structure tends to be broken when it is precompressed.

The density of the compressible closed cell porous material should preferably not be too high as it becomes difficult to compress. It should preferably be between 0.5 and 6.0 lb./cu. ft. (0.008 and 0.096 gm./cm. A density of between 2.0 lb./ cu. ft. and 3.0 lb./ cu. ft. (0.032 and 0.047 gm./cm. is usually suitable for polyvinyl chloride.

In preparing the seal by the process of the invention, the compressible closed cell porous material must be precompressed, that is compressed by at least 5% before inserting it into the zone to be sealed. It should also be compressed without significant damage to the structure of the material, that is for foamed polyvinyl chloride compressed by up to about 25%, e.g. by between 5% and 20%. The exact degrees of compression depends on the compressive strength of the material, the less rigid materials being susceptible to a greater degree of compression. In general, it has been found that the greater the degree of precompression, the greater the elongation at low temperatures.

The process of this invention is conveniently applied in forming fluid-tight seals between heat insulating blocks e.g. those used for lining tanks for housing very cold liquids. The blocks which should be dimensionally stable may have structurally strong front, back and side walls such as of plywood enclosing a porous insulating core such as cores of balsa wood, cork-board, calcium silicate, or a honeycomb core, the honeycombs for example being filled with polyurethane foam or other heat insulating plastic.

The precompressed closed cell porous material e.g. foamed plastic, must be bonded to the blocks (or other material bounding the zone to be sealed) so that the gap between adjacent blocks is effectively sealed. A suitable method of bonding is by gluing e.g. by using epoxy adhesives (e. g. Epikote 828/diethylene triamine (100/10 pt. wt.)), polyester adhesives, or resorcinol glues. Some adhesives contain solvents, which may attack the compressible material, and accordingly care should be taken when selecting a suitable adhesive.

When a container provided with seals according to the invention is filled with cold liquefied gas, the blocks of heat-insulating material will contract and since the piece of for example, foamed plastic is bonded to the blocks, the piece of foamed plastic will come under tension and will be extended. Since the piece of foamed plastic has, however, been installed in the precompressed condition at ambient temperature, the piece of foamed plastic can be extended substantially at low temperature without danger of breakage. The result is that the seal between adjacent blocks of heat-insulating material remains fluid-tight when the container is cooled down to the temperature of the liquefied gas.

In order to demonstrate the better mechanical properties of precompressed closed cell porous material compared with the same material in the un-compressed condition, the following tests were carried out on pieces of blown polyvinyl chloride foam 3.15" x 1.5 x 1.5" (8 x 4 x 4 cm.).

To show the efiect of the degree of compression on the elongation at low temperature, specimens of a different type of polyvinyl chloride foam 4 x 2" X 2" (10 x 5 X 5 cm.) were compressed to different degrees,

Compression, Elongation,

percent percent In these tests the specimens of polyvinyl chloride foam were bonded to 2" (5 cm.) square wood pulling sticks.

Example The gap between adjacent blocks of balsa wood was sealed by inserting polyvinyl chloride foam precompressed by 20%, and stuck to balsa wood by using the combination of Epikote 828 and diethylene triamine in a weight ratio of 100:10. At low temperatures the seal remained fluid-tight and was able to undergo elongations of the order of 5% without fracturing.

I claim:

1. A process for forming a fluid-tight thermal insulating liner for a liquid gas container which comprises (a) affixing to a backing support at ambient temperature a number of rigid blocks of thermal-insulating material, with a small gap between adjacent blocks,

(b) compressing slabs of semi-rigid compressible closed cell foamed plastic by at least 5% from their normal thickness at ambient temperature to at least the thickness of the gap width,

(c) inserting said compressed slabs into the gaps after applying adhesive bonding material between the adjacent surfaces of the slabs and the blocks between which they are inserted to firmly bond the slabs and blocks so as to provide a fluid-tight liner consisting of said blocks with slabs filling all the gaps between them,

((1) the thickness of said slabs in relation to the blocks being such that when the container is filled with lowtemperature liquid gas, the resulting thermal contraction of the blocks and of the slabs changes the condition of the slabs from compression to tension by virtue of said adhesive bonding, without impairing the fluid seal provided by the liner.

2. A process for forming a seal which remains fluidtight at low temperatures in a zone bounded by units of material to be sealed together which comprises substantially filling the zone to be sealed with a compressible closed cell porous material at ambient temperature and in the precompressed condition the degree of compression being between 5 and 25 percent of the original thickness, and adhesively bonding the closed cell porous material firmly to the material bounding said zone.

3. A process as claimed in claim 1 in which the compressible closed cell porous material is a foamed plastic.

4. A process as claimed in claim 3 in which the plastic is a thermoplastic.

5. A process as claimed in claim 3 in which the plastic is polyvinyl chloride.

6. A process as claimed in claim 3 in which the plastic is polystyrene.

7. A process as claimed in claim 3 in which the plastic is polyethylene.

8. A process as claimed in claim 3 in which the plastic is polyurethane.

9. A process as claimed in claim 3 in which the foamed plastic is a material able to be compressed at least 5% without serious damage to the structure.

10. A process as claimed in claim 3 in which the foamed plastic has a compressive strength at 20% compression of between 5 and psi.

11. A process as claimed in claim 3 in which the density of the foamed plastic is between 0.5 and 6.0 lb./ cu. ft.

12. A process for forming a seal which is fluid-tight at low temperatures which comprises filling the gap between adjacent blocks of rigid heat insulating material with compressible closed cell foamed plastic at ambient temperature and in precompressed condition, and adhesively bonding the closed cell foamed plastic firmly to the blocks of heat insulating material.

13. A process as claimed in claim 12 in which the blocks of heat insulating material have a porous insulating core.

14. A process as claimed in claim 12 in which the foamed plastic is foamed polyvinyl chloride.

15. A process as claimed in claim 14 in which an epoxy adhesive is used to bond the closed cell foamed polyvinyl chloride to the blocks of heat insulating material.

16. A process for forming a seal which is fluid-tight at low temperatures which comprises substantially filling the zone to be sealed With a compressible closed cell foamed polyvinyl chloride at ambient temperature and in the precompressed condition, and bonding the foamed polyvinyl chloride firmly to the material bounding said zone.

17. A process as claimed in claim 16 in which poly- 6 vinyl chloride of a density of between 2.0 lb./eu. ft. and 3.0 lb./cu. ft. is used.

18. A process as claimed in claim 16 in which the foamed polyvinyl chloride is precompressed by between 5% and 20%.

References Cited by the Examiner UNITED STATES PATENTS 1,885,391 11/1932 Thompson et al. 9418.2 2,068,035 1/1937 Meyer. 2,179,542 11/1939 Claxton et al. 2209 2,869,336 1/1959 Smidl et al. 2,964,424 12/ 1960 Mast. 3,073,065 1/1963 Alderman et al. 29451 XR 3,074,586 1/1963 Morrison 2209 3,082,483 3/1963 Bickford 264321 3,108,813 10/1963 Brown et al. 29450 XR FOREIGN PATENTS 213,782 2/1961 Austria.

OTHER REFERENCES The Navy Civil Engineer, Preformed Polyurethane Strips, August 1961, pp. 32-35. 5 

1. A PROCESS FOR FORMING A FLUID-TIGHT THERMAL INSULATING LINER FOR A LIQUID GAS CONTAINER WHICH COMPRISES (A) AFFIXING TO A BACKING SUPPORT AT AMBIENT TEMPERATURE A NUMBER O RIGID BLOCKS OF THERMAL-INSULATING MATERIAL, WITH A SMALL GAP BETWEEN ADJACENT BLOCKS, (B) COMPRESSING SLABS OF SEMI-RIGID COMPRESSIBLE CLOSED CELL FOAMED PLASTIC BY AT LEAST 5% FROM THEIR NORMAL THICKNESS AT AMBIENT TEMPERATURE TO AT LEAST THE THICKNESS OF THE GAP WIDTH, (C) INSERTING SAID COMPRESSED SLABS INTO THE GAPS AFTER APPLYING ADHESIVE BONDING MATERIAL BETWEEN THE ADJACENT SURFACES OF THE SLABS AND THE BLOCKS BETWEEN WHICH THEY ARE INSERTED TO FIRMLY BOND THE SLABS 